It appears that the SUMMARY=EMPTY is a problem with some of the entries. Fixing it enabled the import to succeed.

We've also noticed that every instance of ATTENDEE;CN needs to be replaced with ATTENDEE;RSVP=FALSE, as these are entries where the user hasn't RSVP'd to an appointment yet, and Zimbra doesn't seem to like these on the import without specifying the RSVP status.